Your paper will first be read by one of the journal’s in-house editors, who will decide whether it should be sent for peer review. The majority of papers submitted are rejected at this stage. Unfortunately most journals can only accept a very low percentage of the submission; for example The Lancetaccepts less than 10% of papers at this stage of the review process.
Do not be discouraged if your paper is returned. It is most likely due to an editorial decision; perhaps they have just published a few papers in your area, perhaps they have a build-up of good papers, or perhaps your paper was not appropriate for the journal.
Keep in mind some alternative journals.